The owners of other websites I know feel very troubled by the idea of letting anyone access user IP data, or even about storing it. I know storing user IPs for all visits ever, indefinitely, was a perfectly normal thing to do 20 years ago, but in the times of GDPR lawsuits, it's more like you have to hire a data privacy specialist who has to approve every bit of user information stored.

Sure, it was 10 years ago and long before GDPR, not sure if mods have there now information about actual IP. But point stands that using user IP to prevent having multiple accounts / ban evasion / DDOS attacks / malfunctioning scripts that send hundreds of requests per second etc. is normal and part of server security. Unless you go on anonymous network where you don't have identification and all users write as "user" - logs will be collected for the purpose of website security and any potential requests made by the authorities. GDPR does not mean website owners lost ability to identify security threats based on IP, as it can be considered private information.

So in case sensualshakti gets into a car crash and can't use both arms for some time, would asking another SG user to update a Humble Bundle thread in the Deals forum on her account lead to a permaban for both?

Yes. What should be done is write post "Hey guys! sensualshakti will not be able to make new posts while they recover. In the mean time I will be updating deals". Thread categories do not "belong" to anyone. Before it was rachellove who was updating deals section. When she left someone else took over. Not start to use rachel account to keep posting thread under her name.

(Sharing an account in the way that it's essentially another person's alt and the whole thing becomes an account farm, or entering each other's GAs to make a fake little 5-user distribution ring, sure, sure. Those are all clear, and unfortunately some of them really happened. I'm just asking about forum posts unrelated to gaining games or increasing CV or any of those things.)

There is no "good" and "bad" account sharing. All is not allowed. Users can use only their SG account, connected to their own Steam account. And create content (write discussions, posts, giveaways) only themselves.

Auto joiners break entirely different point of the guidelines that talk about using automated means to enter giveaways, as it gives them advantage over people that have "breaks" in using the site when they sleep, eat, go to school or work. Which brings up another point:

theoretically having someone else join giveaways while you're at school or work or what is harmless

This is not allowed as both account sharing and partially as "auto joining". And is mayor reason why sharing accounts is not allowed. People can't exchange log in details to "be always online" by juggling few accounts and enter giveaways for other people, when rightful owner is sleeping / working / on holiday.